<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 2个类选择器：0020 */
        .box .child {
            background-color: blueviolet;
        }

        /* 类选择器 + 标签选择器：0011 */
        .box>div {
            width: 100px;
            height: 100px;
            background-color: blue !important;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="child" style="background-color: red;"></div>
    </div>
</body>

</html>